Metrology systems of the type mentioned at the beginning are known from US 2013/0063716 A1, DE 102 20 815 A1, DE 102 20 816 A1 and US 2013/0083321 A1. EP 2 506 061 A1 discloses a projection optical unit for a projection exposure apparatus for producing semiconductor devices that uses an aperture stop in which the diameter of the stop in two mutually perpendicular directions differs by more than 10%. DE 10 2010 040 811 A1 describes an anamorphic projection optical unit. US 2008/0036986 A1 describes a projection exposure apparatus.